This invention relates to a light beam scanning apparatus for making laser light beams to scan and expose the surface of a single photosensitive drum simultaneously and thereby forming a single electro-static latent image on the photosensitive drum, an image forming apparatus, such as a digital copying machine or a laser printer, using the light beam scanning apparatus, and a method of adjusting the image forming apparatus.
In recent years, various digital copying machines have been developed which form images by, for example, scanning exposure by laser light beams (hereinafter, referred to as light beams) and electronic photographic processing.
To step up the image forming speed, the multi-beam digital copying machines have recently been developed. In this type of digital copying machine, more than one light beam is generated and they are made to scan in units of lines simultaneously.
The multi-beam digital copying machine is provided with an optical system unit acting as a light beam scanning apparatus. The optical system unit is composed primarily of semiconductor laser oscillators for generating laser beams, a polyhedral rotating mirror, such as a polygon mirror, and optical-path control means. The polyhedral rotating mirror reflects the laser beams emitted from the laser oscillators toward a photosensitive drum to make each laser beam to scan the surface of the photosensitive drum. The optical-path control means is composed of a collimator lens, an f-.theta. lens, and a galvanomirror for shifting the passing position of each light beam over the photo-sensitive drum in the sub-scanning direction.
With the configuration of a conventional optical system unit, it is very difficult to control the positional relationship between light beams on the surface of a photosensitive drum (the scanning surface) or to bring the passing positions of light beams into an ideal positional relationship. To achieve this, not only very high-accuracy parts and assembly but also very high-accuracy light-beam passing position control are needed, which leads to a rise in the cost of the apparatus.
Even when the light beams have been controlled so as to have an ideal positional relationship, noise can interfere with the driving lines for a light beam actuator, such as a galvanomirror, after the control, which will possibly lead to an erroneous operation of the galvanomirror, permitting the passing positions of the light beams to deviate from the target position.
In addition, there is a possibility that the passing positions of the light beams will depart from the target position due to vibration made by the image forming apparatus or externally applied vibration.
Furthermore, noise can interfere with the output of a sensor for sensing the passing position of a light beam, which will possibly perform control on the basis of erroneous information on the light beam position.
Moreover, in a light beam actuator, such as a galvanomirror, the hysteresis or sensitivity generally varies. Such variations in the hysteresis or sensitivity have an adverse effect on control of the passing positions of the light beams. For example, there is a possibility that control will vibrate and the light beams will not converge in the control target area.